WorldCat is a union catalog that itemizes the collections of 15,600 libraries in 107 countries that participate in the OCLC global cooperative. It is operated by OCLC, Inc. The subscribing member libraries collectively maintain WorldCat's database, the world's largest bibliographic database. The database includes other information sources in addition to member library collections. OCLC makes WorldCat itself available free to libraries, but the catalog is the foundation for other subscription OCLC services (such as resource sharing and collection management). WorldCat is used by the general public and by librarians for cataloging and research.
OCLC was founded in 1967 under the leadership of Fred Kilgour. That same year, OCLC began to develop the union catalog technology that would later evolve into WorldCat; the first catalog records were added in 1971.
In 2003, OCLC began the "Open WorldCat" pilot program, making abbreviated records from a subset of WorldCat available to partner web sites and booksellers, to increase the accessibility of its subscribing member libraries' collections.
In October 2005, the OCLC technical staff began a wiki project, WikiD, allowing readers to add commentary and structured-field information associated with any WorldCat record. WikiD was later phased out, although WorldCat later incorporated user-generated content in other ways.
In 2006, it became possible for anyone to search WorldCat directly at its open website, not only through the subscription FirstSearch interface where it had been available on the web to subscribing libraries for more than a decade before. Options for more sophisticated searches of WorldCat have remained available through the FirstSearch interface.
In 2007, WorldCat Identities began providing pages for 20 million "identities", which are metadata about names—predominantly authors and persons who are the subjects of published titles.
In 2017, OCLC's WorldCat Search API was integrated into the cite tool of Wikipedia's VisualEditor, allowing Wikipedia ors to cite sources from WorldCat easily.
Beginning in 2017, OCLC and the Internet Archive have collaborated to make the Internet Archive's records of digitized books available in WorldCat.
As of July 2020, WorldCat contained almost 500 million bibliographic records in 483 languages, representing over 3 billion physical and digital library assets, and the WorldCat persons dataset (mined from WorldCat) included over 100 million people.
WorldCat contains records in MAchine Readable Cataloging (MARC) format contributed by library catalogers worldwide who use OCLC as a cataloging tool, and these MARC format records can also be downloaded into other libraries' local catalog systems. This allows libraries to find and download records for materials they are adding to their local catalog, without having to undergo the lengthy process of creating a new catalog entry from scratch for each new item.
WorldCat operates on a batch processing model rather than a real-time model. That is, WorldCat records are synchronized at intermittent intervals with the underlying library catalogs instead of real-time or every day. Consequently:
WorldCat shows that a particular item is owned by a particular library but does not provide that library's call number.
WorldCat does not indicate whether an item is currently borrowed, lost, undergoing restoration or repair, or moved to storage not directly accessible to patrons (thereby forcing interested patrons to submit a retrieval request and wait).
Furthermore, WorldCat does not show whether a library owns multiple copies of a particular title.
As an alternative, WorldCat allows participating institutions to add direct links from WorldCat to their own catalog entries for a particular item, which enables the user to determine its real-time status. However, this still requires users to open multiple Web pages, each pointing to a different online public access catalog with its own distinctive user interface design (which places item status in a different portion of the Web browser display), until they can locate a catalog entry that shows the item is currently available at a particular library.
Library contributions to WorldCat are made via the Connexion computer program, which was introduced in 2001; its predecessor, OCLC Passport, was phased out in May 2005.
^ abMargalit Fox (August 2, 2006). "Frederick G. Kilgour, Innovative Librarian, Dies at 92". The New York Times. Retrieved December 22, 2009. Frederick G. Kilgour, a distinguished librarian who nearly 40 years ago transformed a consortium of Ohio libraries into what is now the largest library cooperative in the world, making the catalogs of thousands of libraries around the globe instantly accessible to far-flung patrons, died on Monday in Chapel Hill, N.C. He was 92.
^Bertot, John Carlo; Berube, Katy; Devereaux, Peter; Dhakal, Kerry; Powers, Stephen; Ray, Jennie (April 2012). "Assessing the usability of WorldCat Local: findings and considerations". The Library Quarterly. 82 (2): 207–221. doi:10.1086/664588. JSTOR10.1086/664588. S2CID61287720. Breeding  also makes the following observations about the benefits of the search system: the presence of a more visually appealing interface; the grouping of related material; faceted navigation; and the capability for user-generated content (e.g., reviews). Eden  also refers to the advantages of user-generated content possible in WCL...
^Prucha, Francis Paul (1994). "National online library catalogs". Handbook for research in American history: a guide to bibliographies and other reference works (2nd ed.). Lincoln: University of Nebraska Press. pp. 25–27. ISBN0803237014. OCLC28018047. Online Computer Library Center has developed two new programs. One is called EPIC, a new command-driven full online service with sophisticated searching features, including subject searches, intended for librarians and other experienced users. The other, designed for end-users, is FirstSearch, which contains the database materials found in EPIC or subsets of them but has a menu interface that nonspecialists find easy to use. Both EPIC and FirstSearch make available the full OCLC Online Union Catalog (called WorldCat in FirstSearch), but they also function as online database services, offering their users a wide array of other databases.